Experimental Determination of Crack Initiation and Crack Damage of Two Granites

Mineralogical composition and microstructural features influence rock strength and their failure mechanisms. Crack initiation (CI) and crack damage (CD) are important processes of the failure mechanisms of rocks. Since CI is the lower limit of the long term strength of rocks at low confinement, this study is mainly concerned with triaxial tests and the determination of CI and CD using different methods. A triaxial cell has been designed to examine spalling behavior of magmatic rocks under low confinement, at minor principal stresses up to 2.5 MPa. Two granites with different grain sizes were tested. Methods to determine CI and CD include the calculation of circumferential, volumetric and crack volumetric strain. In addition, acoustic emission monitoring provides the onset of CI and CD. Uniaxial compressive tests including primary wave velocity measurements as well as acoustic emission were used to supplement the dataset. The wave velocities also give an indirect determination of CI and CD as the initiation of cracks reduces the speed of the waves. This study aims to compare the different methods determining CI and CD and to connect the results with mineralogical and microstructural features. Orientated thin sections were prepared to identify the mineralogical composition. Furthermore, the growing influence of confining pressure from 0.1 to 2.5 MPa on rock strength was analyzed.